Systematic ionospheric electron density tilts (SITs) at mid-latitudes and their associated HF bearing errors

Abstract

The bearings of a number of HF transmissions recorded continuously over many days are employed to investigate the large-scale horizontal gradients of electron density in the mid-latitude ionosphere (systematic ionospheric tilts or SITs). Although previous authors have explained these SITs and their associated bearing errors solely in terms of a dependence of ionospheric tilt on the variation of solar zenith angle, our observations indicate that plasma transport processes must also be taken into account. A 3D ray tracing technique is employed in conjunction with an ionospheric model based on predicted vertical-sounder data to simulate the experimental observations. Good agreement is obtained between predicted and observed bearing errors for south-to-north (S-N) transmission paths, although poor agreement is achieved for east-to-west (E-W) paths. The latter is due to the inability of our ionospheric model to yield accurate N-S ionospheric gradient information. Better agreement between computed and observed bearing errors for E-W paths is achieved by ray tracing through an ionospheric model based on measured median vertical-sounder data. A major limiting factor in the accurate simulation of SIT-induced bearing error is the day-to-day variation of ionospheric tilts.